In a 2-quart pot, add oil and brown the meat with the onion, stirring often. Once the meat is cooked, add Mamacita's Salsa Fresca, chili powder, salt, and pepper. Cook for approximately 5-10 minutes until the onions are soft and translucent. Add both beans and tomatoes, then stir. Cook for 20 minutes on low heat. If you like more liquid, add a little water to the mix. Serve with a garnish of sour cream and chopped green onion.